High performance heterocycles

ABSTRACT

Linear or multidimensional, crosslinking, solvent resistant oxazole, thiazole, or imidazole (i.e., heterocycle) oligomers and blends of the crosslinking oligomers and noncrosslinking comparable polymers are described. The oligomers are prepared by reacting tetraamines, diaminodiols, or diaminothiols (i.e. four-functional compounds) with poly-carboxylic acid halides, and crosslinking phenylimide end cap monomers in a suitable solvent under an inert atmosphere.

REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 07/116,592, filedNov. 3, 1987 now U.S. Pat. No. 4,965,336, which is acontinuation-in-part application based upon Ser. No. 06/816,490, filedJan. 6, 1986, now abandoned which was a continuation-in-part applicationbased upon Ser. No. 06/651,826, filed Sep. 18, 1984, now abandoned.

TECHNICAL FIELD

The present invention relates to linear or multidimensional, solventresistant, crosslinkable oligomers that include oxazole, thiazole, orimidazole linkages along the oligomer backbone between mono- ordifunctional crosslinking end-cap phenylimides of the general formula:##STR1## and to their method of manufacture.

BACKGROUND ART

Recently, chemists have sought to synthesize oligomers for highperformance advanced composites suitable for aerospace applications.These composites should exhibit solvent resistance, be tough, impactresistant, and strong, be easy to process, and be thermoplastic.Oligomers andd composites that have thermooxidative stability, and,accordingly can be used at elevated temperatures, are particularlydesirable.

While epoxy-based composites are suitable for many applications, theirbrittle nature and susceptibility to degradation make them inadequatefor many aerospace applications, especially those applications whichrequire thermally stable, tough composites. Accordingly, research hasrecently focused upon polyimide composites to achieve an acceptablebalance between thermal stability, solvent resistance, and toughness.The maximum use temperatures of conventional polyimide composites, suchas PMR-15, are still only about 600°-625° F., since they have glasstransition temperatures of about 690° F.

Linear polysulfone, polyether sulfone, polyester, and polyamide systemsare also known, but each of these systems fails to provide as highthermal stability as is required in some aerospace applications.

There has been a progression of polyimide sulfone compounds synthesizedto provide unique properties or combinations of properties. For example,Kwiatkowski and Brode synthesized maleic-capped, linear polyarylimidesas disclosed in U.S. Pat. No. 3,839,287. Holub and Evans synthesizedmaleic- or nadic-capped, imido-substituted polyester compositions asdisclosed in U.S. Pat. No. 3,729,446. We synthesized thermally stablepolysulfone oligomers as disclosed in U.S. Pat. No. 4,476,184 or U.S.Pat. No. 4,536,559, and have continued to make advances withpolyetherimidesulfones, polybenzoxazolesulfones (i.e., heterocycles),polybutadienesulfones, and "star" or "star-burst" multidimensionaloligomers. We have shown surprisingly high glass transition temperaturesand desirable physical properties in many of these oligomers and theircomposites, without losing ease of processing.

Multidimensional oligomers, such as disclosed in our copendingapplications U.S. Ser. Nos. 726,258; 810,817; and 000,605 now U.S. Pat.No. 4,228,092, are easier to process than many other advanced compositeoligomers since they can be handled at lower temperatures. Upon curing,however, the unsaturated phenylimide end caps crosslink so that thethermal resistance of the resulting composite is markedly increased withonly a minor loss of stiffness, matrix stress transfer (impactresistance), toughness, elasticity, and other mechanical properties.Glass transition temperatures above 950° F. are achievable.

Commercial polyesters, when combined with well-known diluents, such asstyrene, do not exhibit satisfactory thermal and oxidative resistance tobe useful for aircraft or aerospace applications. Polyarylesters areunsatisfactory, also, since the resins often are semicrystalline whichmakes them insoluble in laminating solvents, intractable in fusion, andsubject to shrinking or warping during composite fabrication. Thosepolyarylesters that are soluble in conventional laminating solventsremain so in composite form, thereby limiting their usefulness instructural composites. The high concentration of ester groupscontributes to resin strength and tenacity, but also makes the resinsusceptible to the damaging effects of water absorption. High moistureabsorption by commercial polyesters can lead to distortion of thecomposite when it is loaded at elevated temperature.

High performance, aerospace, polyester advanced composites, however, canbe prepared using crosslinkable, end-capped polyester imide ethersulfone oligomers that have an acceptable combination of solventresistance, toughness, impact resistance, strength, ease of processing,formability, and thermal resistance. By including Schiff base(--CH═N--), imidazole, thiazole, or oxazole linkages in the oligomerchain, the linear, advanced composites formed with polyester oligomersof our copending application U.S. Ser. No. 726,259 can havesemiconductive or conductive properties when appropriately doped.

Conductive and semiconductive plastics have been extensively studied(see, e.g., U.S. Pat. Nos. 4,375,427; 4,338,222; 3,966,987; 4,344,869;and 4,344,870), but these polymers do not possess the blend ofproperties which are essential for aerospace applications. That is, theconductive polymers do not possess the blend of (1) toughness, (2)stiffness, (3) elasticity, (4) ease of processing, (5) impact resistance(and other matrix stress transfer capabilities), (6) retention ofproperties (over a broad range of temperatures), and (7) hightemperature resistance that is desirable on aerospace advancedcomposites. These prior art composites are often too brittle.

Thermally stable multidimensional oligomers having semiconductive orconductive properties when doped with suitable dopants are also knownand are described in our copending applications (including U.S. Ser. No.773,381 abandoned to Lubowitz, Sheppard, and Torre). The linear arms ofthe oligomers contain conductive linkages, such as Schiff base(--N═CH--) linkages, between aromatic groups. Sulfone and ether linkagesare interspersed in the arms. Each arm is terminated with a mono- ordifunctional end cap (i.e., a radical having one or two crosslinkingsites) to allow controlled crosslinking upon heat-induced orchemically-induced curing.

Polyamides of this same general type are described in our copendingpatent application U.S. Ser. No. 061,938 U.S. Pat. No. 4,876,328;polyetherimides, in U.S. Ser. No. 016,703 U.S. Pat. No. 4,851,495; andpolyamideimides, in U.S. Ser. No. 092,740 abandoned.

SUMMARY OF THE INVENTION

The present invention relates to linear or multidimensional oxazole,thiazole, and imidazole (i.e., heterocycle) oligomers, particularlybenzoxazole, benzothiazole, and benzimidazole oligomers, capped withmono- or difunctional end-cap monomers (i.e., monomers having one or twocrosslinking sites) to achieve superior thermal stability whileretaining desirable strength and physical properties.

The oligomers are usually prepared by the condensation of:

(a) 2 moles of a phenylimide carboxylic acid halide end-cap monomer ofthe general formula: ##STR2## wherein D=an unsaturated hydrocarbonradical;

i=1 or 2; and

φ=phenyl

(b) n moles of a diacid halide particularly an aromatic dicarboxylicacid halide having a plurality of aryl groups intermediately linked by"sulfone" linkages and terminal carboxylic acid halide functionalitiesattached to the aryl groups; and

(c) (n+1) moles of a four-functional compound of the formula: ##STR3##(such as a diaminodihydroxybenzene) wherein R is an hydrocarbon radical(preferably, an aromatic radical, if the highest thermal stability issought); Y=--OH, --NH₂, or --SH; and the amine functionalities (--NH₂)are not substituted on the same carbon atom as the Y substituents.

The end-cap monomer preferably is selected from the group consisting of:

    [D].sub.i --φ--COX

wherein ##STR4## R₁ =lower alkyl, aryl, substituted aryl (includinghydroxyl or halo-substituents), lower alkoxy, aryloxy, halogen, ormixtures thereof (preferably lower alkyl);

X=halogen, preferably Cl;

G=--SO₂ --, --S--, --O--, or --CH₂ --;

i=1 or 2;

j=0, 1, or 2;

φ=phenyl;

T=methallyl or allyl; and

Me=methyl.

Preferred end-cap monomers are the phenylimide acid halides wherein Dincludes a radical selected from: ##STR5## wherein R" is hydrogen orlower alkyl.

Blended oligomers are prepared to include the crosslinking oligomers andat least one comparable, noncrosslinking polymer. The polymer generallyhas a substantially identical backbone to the oligomer, but isterminated (or quenched) with a monomer that is unable to crosslink.Accordingly, the comparable polymer is generally prepared by condensing:

(a) 2 moles of an acid halide end-cap quenching monomer;

(b) n moles of the diacid halide of the crosslinking oligomer;

(c) (n+1) moles of the four-functional compound of the crosslinkingoligomer.

A suitable monomer for quenching the polymerization reaction for thecomparable oligomer is benzoic acid halide ##STR6##

Of course, the crosslinking oligomers can also be prepared by thecondensation of:

(a) 2 moles of a suitable phenylimide amine, phenol, or sulfhydryl(i.e., thio) monomer;

(b) n moles of a four-functional compound; and

(c) (n+1) moles of a suitable diacid halide.

The comparable polymer could also include the analogous backbone andcould be quenched with a phenol or suitable thio- or amino-monomer (suchas aniline).

Generally, the four-functional compound is selected from the groupconsisting of:

dihydroxybenzidine;

dimercaptobenzidine;

2,6-diamino-3,5-dihydroxybenzene;

2,6-diamino-3,5-dimercaptobenzene; or

diaminobenzidine.

Heterocyle oligomers of this general type are easily processed intoprepregs and composites. The composites (or laminates) are chemically,thermally, and dimensionally stable at relatively high temperatures andexhibit solvent-resistance.

Multidimensional oligomers can be prepared by reacting thefour-functional compounds and phenylimide acid halide end-cap monomerswith an aromatic hub having 3 or more reactive acid halidefunctionalities, such as a compound of the formula: ##STR7## whereinAr=an aromatic moiety of valence w, and

w=an integer greater than or equal to 3, and generally 3 or 4.

The hub (Ar) may be a residue of cyuranic acid or an imide-acid compoundformed by reacting, for example, triaminobenzene with phthalic acidanhydride or a corresponding acid anhydride. The arms of themultidimensional oligomers can be extended by adding diacid halides tothe reaction mixture, as will be understood. Corresponding ether/acidhubs can be prepared by condensing a phenolic hub, like phloroglucinolwith nitrobenzoic acid or with nitrophthalic acid. Blends ofmultidimensional oligomers and corresponding polymers can also beprepared.

BEST MODE CONTEMPLATED FOR CARRYING OUT THE INVENTION

The crosslinking oligomers of the present invention are oxazoles,thiazoles, or imidazoles (i.e., heterocycles) generally prepared by thecondensation of:

(a) 2 moles of a phenylimide carboxylic acid halide end-cap monomer;

(b) n moles of a diacid halide, particularly an aromatic dicarboxylicacid halide having a plurality of aryl groups intermediately linked by"sulfone" (i.e., electronegative) linkages and terminal acid halidefunctionalities attached to aryl groups; and

(c) (n+1) moles of a four-functional compound of the formula: ##STR8##wherein R is a hydrocarbon radical (preferably an aromatic radical),Y=--OH, --SH, or --NH₂, and the Y and --NH₂ are on separate carbonatoms. Generally, the amine and Y are on adjacent carbon atoms of anaromatic ring. The four-functional compound, accordingly, is generallyselected from the group consisting of:

dihydroxybenzidine;

dimercaptobenzidine;

2,6-diamino-3,5-dihydroxybenzene;

2,6-diamino-3,5-dimercaptobenzene; or

diaminobenzidine.

The end-cap monomer generally is selected from the group consisting of:

    [D].sub.i --φ--COX

wherein ##STR9## R₁ =lower alkyl, aryl, substituted aryl (includinghydroxyl or halo-substituents), lower alkoxy, aryloxy, halogen, ormixtures thereof (preferably lower alkyl);

X=halogen;

φ=phenyl;

G =--0--,--S--, --SO₂ -- or --CH₂ --;

i=1 or 2;

j=0, 1, or 2;

T=methallyl or allyl; and

Me=methyl.

These end-cap monomers have hydrocarbon unsaturation to provide one orincludes a radical selected from: two crosslinking sites. For thehighest thermal stability, D ##STR10## wherein R" is hydrogen or loweralkyl.

The reaction is generally carried out at an elevated temperature underan inert atmosphere (dry N₂ purge) in a suitable solvent including anexcess of base (KOH or NaOH) to reduce the possibility of undesirableside reactions that might otherwise occur in an acidic solution. Usuallyabout 10% excess base is added, based upon the molar quantities of thereactants.

The dicarboxylic acid halide (or dicarboxylic acid) may include anaromatic chain segment selected from the group consisting of:

(a) phenyl;

(b) napthyl;

(c) biphenyl;

(d) a polyaryl "sulfone" divalent radical of the general formula:##STR11## wherein D=--S--, --O--, --CO--, --SO₂ --, --(CH₃)₂ C--,--(CF₃)₂ C--, or mixtures thereof throughout the chain; or

(e) a divalent radical having conductive linkages, illustrated by Schiffbase compounds selected from the group consisting of: ##STR12## whereinR is selected from the group consisting of: phenyl; biphenyl; naphthyl;or a divalent radical of the general formula: ##STR13## wherein W=--SO₂-- or --CH₂ --; and q=0--4; or (f) a divalent radical of the generalformula: ##STR14## wherein R¹ =a C₂ to C₁₂ divalent aliphatic,alicyclic, or aromatic radical, and, preferably, phenyl (as described inU.S. Pat. No. 4,556,697).

Thiazole, oxazole, or imidazole linkages, especially between arylgroups, may also be used in the conductive or semiconductive oligomers,instead of the Schiff base linkages. The oligomers being heterocycles,may be semiconductive upon doping even without incorporating additionalconductive linkages.

The diacid halide preferably is an aromatic dicarboxylic acid selectedfrom the group consisting of: ##STR15## wherein q is an electronegative("sulfone") group, preferably --CO--, --S--, --(CF₃)₂ C--, or --SO₂,and, generally, --CO--, --SO₂ --; and m equals a small integer generallyfrom 1-5.

Preferred diacid halides include: ##STR16##

Schiff base dicarboxylic acids and diacid halides can be prepared by thecondensation of aldehydes and aminobenzoic acid (or other amine acids)in the general reaction scheme: ##STR17## or similar syntheses.

Other diacid halides that can be used, but that are not preferred, aredisclosed in U.S. Pat. No. 4,504,632, and include:

adipylchloride,

malonyl chloride,

succinyl chloride,

glutaryl chloride,

pimelic acid dichloride,

suberic acid dichloride,

azelaic acid dichloride,

sebacic acid dichloride,

dodecandioic acid dichloride,

phthaloyl chloride,

isophthaloyl chloride,

terephthaloyl chloride,

1,4-naphthalene dicarboxylic acid dichloride, and

4,4'-diphenylether dicarboxylic acid dichloride.

Polyaryl or aryl diacid halides are preferred to achieve the highestthermal stabilities in the resulting oligomers and composites becausealiphatic bonds are not as thermally stable as aromatic bonds.Particularly preferred compounds include intermediate "sulfone" (i.e.electronegative) linkages to improve the toughness of the resultingoligomers. For purposes of this description, "sulfone" linkages shouldbe understood to include --SO₂ --, --S--, --CO--, and --(CF₃)₂ C--,unless clearly limited to only --SO₂ --.

Suitable diacid halides include compounds made by reacting nitrobenzoicacid with a bisphenol (i.e., dihydric phenol, dialcohol, or diol). Thebisphenol is preferably selected from the group consisting of:

2,2-bis-(4-hydroxyphenyl)-propane (i.e., bisphenol-A);

bis-(2-hydroxyphenyl)-methane;

bis-(4-hydroxyphenyl)-methane;

1,1-bis-(4-hydroxyphenyl)-ethane;

1,2-bis-(4-hydroxyphenyl)-ethane;

1,1-bis-(3-chloro-4-hydroxyphenyl)-ethane;

1,1-bis-(3,5-dimethyl-4-hydroxyphenyl)-ethane;

2,2-bis-(3-phenyl-4-hydroxyphenyl)-propane;

2,2-bis-(4-hydroxynaphthyl)-propane

2,2-bis-(4-hydroxyphenyl)-pentane;

2,2-bis-(4-hydroxyphenyl)-hexane;

bis-(4-hydroxyphenyl)-phenylmethane;

bis-(4-hydroxyphenyl)-cyclohexylmethane;

1,2-bis-(4-hydroxyphenyl)-1,2-bis-(phenyl)-ethane;

2,2-bis-(4-hydroxyphenyl)-1-phenylpropane;

bis-(3-nitro-4-hydrophenyl)-methane;

bis-(4-hydroxy-2,6-dimethyl-3-methoxyphenyl)-methane;

2,2-bis-(3,5-dichloro-4-hydroxyphenyl)-propane;

2,2-bis-(3-bromo-4-hydroxyphenyl)-propane;

or mixtures thereof, as disclosed in U.S. Pat. No. 3,262,914. Bisphenolshaving aromatic character (i.e., absence of aliphatic segements), suchas bisphenol-A, are preferred.

The bisphenol may be in phenate form, or a corresponding sulfyhydryl canbe used. Of course, mixtures of bisphenols and disulfhydryls can beused.

Other suitable bisphenols are described in our copending applicationsSer. Nos. 016,703 U.S. Pat. No. 4,851,495 and 726,258 pending; or inU.S. Pat. Nos. 4,584,364; 4,661,604; 3,262,914, or 4,611,048.

While bisphenol-A is preferred (because of cost and availability), theother bisphenols can be used to add rigidity to the oligomer withoutsignificantly increasing the average formula weight, and therefore, canincrease the solvent resistance. Random or block copolymers arepossible.

Bisphenols of the type described are commercially available. Some may beeasily synthesized by reacting dihalogen intermediate with bis-phenates,such as the reaction of 4,4'-dichlorophenyl-sulfone with bis(disodiumbiphenolate). Preferred dihalogens in this circumstance are selectedfrom the group consisting of: ##STR18## wherein X=halogen, preferablychlorine; and

q=--S--, --SO₂ --, --CO--, --(CH₃)₂ C--, and --(CF₃)₂ C--, andpreferably either --SO₂ -- or --CO--.

The heterocycle oligomers of the present invention can also be preparedby the condensation of:

(a) 2 moles of a crosslinking phenylimide amine, phenol, or sulfhydrylend-cap monomer;

(b) n moles of the four functional compound; and

(c) (n+1) moles of a suitable diacid halide.

In this case, the end-cap monomer generally has the formula:

    [D].sub.i --φ--Y

wherein D, i, and φ are as previously defined and Y=--OH, --SH, or--NH₂.

Blends can improve impact resistance of composites without causing asignificant loss of solvent resistance. The blends comprise mixtures ofone or more crosslinkable oligomer and one or more polymer that isincapable of crosslinking. Generally, the blends comprise substantiallyequimolar amounts of one polymer and one oligomer having substantiallyidentical backbones. The cross-linkable oligomer and comparable polymercan be blended together by mixing mutually soluble solutions of each.While the blend is preferably equimolar in the oligomer and polymer, theratio of the oligomer and polymer can be adjusted to achieve the desiredphysical properties.

Although the polymer in such a blend usually has the same lengthbackbone as the oligomer, the properties of the composite formed fromthe blend can be adjusted by altering the ratio of formula weights forthe polymer and oligomer. The oligomer and polymer generally havesubstantially identical repeating units, but the oligomer and polymermerely need be compatible in the solution prior to sweeping out as aprepreg. Of course, if the polymer and oligomer have identicalbackbones, compatibility in the blend is likely to occur. Blends thatcomprise relatively long polymers and relatively short oligomers (i.e.,polymers having higher average formula weights than the oligomers) priorto curing are preferred, since, upon curing, the oligomers willeffectively increase in MW by crosslinking.

In synthesizing the comparable polymers, quenching end caps can beemployed, if desired, to regulate the polymerization of the comparablepolymer, so that it has an average formula weight substantiallyidentical with the crosslinkable oligomer. For thermal stability, anaromatic compound, such as aniline or benzoic acid chloride, ispreferred to quench the synthesis.

Solvent resistance may decrease markedly if the comparable polymer isprovided in large excess to the crosslinkable oligomer in the blend.

The blends will generally comprise a mixture of a heterocycle oligomerand the same heterocycle polymer (i.e., oxazole oligomer and oxazolepolymer). The polymer may, however, be a different heterocycle, such asan imide, imidazole, or thiazole. The mixture may include several typesof oligomers or several types of polymers, such as a three componentmixture of an oxazole oligomer, a thiazole oligomer, and an imidazolepolymer.

The blends may be semi-interpenetrating networks of the general typedescribed by Egli et al. "Semi-Interpenetrating Networks of LARC-TPI"available from NASA-Langley Research Center.

Because the oligomers synthesized in accordance with this inventiongenerally have appreciable molecular weight between the reactive(crosslinking) groups, the oligomers will retain sufficient plasticityto be processible during fabrication prior to crosslinking of the endcaps to thermoset composites. If possible, thermoplastic formulationswith high molecular weights should be synthesized so long as theoligomers retain the necessary solubility. The oligomers preferably haveMWs (i.e., average formula weights) between about 5000-40,000, and, morepreferably, between about 15,000-25,000. Thermosetting heterocycleoligomers of the present invention generally will have average formulaweights of between about 500-5000. Mixtures of oligomers havingmolecular weights within these ranges may also be used, for example, amixture of an oligomer having a molecular weight of about 1,000 with anoligomer having a molecular weight of about 20,000, or a mixture of anoligomer with a molecular weight of about 5,000 with an oligomer havinga molecular weight of about 10,000 or about 20,000. Within the describedranges, the oligomers can be crosslinked to form solvent resistantcomposites of high thermal stability suitable for many aerospaceapplications. The oligomers, however, are relatively soluble, and,therefore, may be easily processed into prepregs by conventional steps.

Solubility of the oligomers becomes an increasing problem as chainlength increases. Therefore, shorter chains are preferred, if theresulting oligomers remain processible. That is, the chains should belong enough to yield thermoplastic characteristics to the oligomers butshort enough to keep the oligomers soluble during the reaction sequence.

Linear heterocycle oligomers preferably are synthesized using a diacidhalide selected from the group consisting of: ##STR19## wherein q isselected from the group consisting of --(CF₃)₂ C--, --SO₂ --, --S--, or--CO--; X₁ is selected from the group consisting of --O-- or --SO₂ --;E, E₁, E₂ and E₃ each represent substituent groups selected from thegroup consisting of halogen, alkyl groups having 1 to 4 carbon atoms,and alkoxy groups having 1 to 4 carbon atoms, and "a," "b," "c," and "d"are all integers having values from 0 to 4.

The compound: ##STR20## is particularly preferred, especially if theend-cap monomer is either: ##STR21##

Multidimensional oligomers may be synthesized using an aromatic hub,such as cyuranic acid (or its acid halide), the four functionalcompounds, and the acid halide end-cap monomers. The oligomers have thegeneral formula:

    Ar--M).sub.w

wherein

Ar=the aromatic hub residue;

M=a monovalent radical having at least one heterocyclic (oxazole,thiazole, or imidazole) linkage and at least one, terminal, crosslinkingfunctionality; and

w=an integer greater than or equal to 3, and preferably 3 or 4.

The chains within each arm (M) can be extended by including diacidhalides in the reaction mixture.

In multidimensional oligomers, an aromatic hub includes a plurality ofrays or spokes radiating from the hub in the nature of a star to providemultidimensional crosslinking through suitable terminal groups with agreater number (i.e. higher density) of crosslinking bonds than lineararrays provide. Usually the hub will have three radiating chains to forma "Y" pattern. In some cases, four chains may be used. Including morechains leads to steric hindrance as the hub is too small to accommodatethe radiating chains. A trisubstituted phenyl hub is highly preferredwith the chains being symmetrically placed about the hub. Biphenyl,naphthyl, or azaline (e.g., melamine) may also be used as the hubradical along with other aromatic moieties, if desired.

Triazine derivatives can be used as the hub. These derivatives aredescribed in U.S. Pat. No. 4,574,154 and have the general formula:##STR22## wherein R₂ is a divalent hydrocarbon residue containing 1-12carbon atoms (and, preferably, ethylene) by reacting the aminefunctionalities with phthalic acid anhydride to form arms that includeimide linkages and terminal acid functionalities (that can be convertedto acid halides, if desired). The triazine derivatives of U.S. Pat. No.4,617,390 (or the acid halides) can also be used as the hub.

Hubs suitable for making multidimensional, heterocycle oligomers of thepresent invention can be made by reacting polyol aromatic hubs, such asphloroglucinol, with nitrobenzoic acid or nitrophthalic acid to formether linkages and active, terminal carboxylic acid functionalities. Thenitrobenzoic acid products would have three active sites while thenitrophthalic acid products would have three active sites while thenitrophthalic acid products would have six; each having the respectiveformula:

    φ--O--φ--COOH].sub.3 or φ--0--φ--(COOH).sub.2 ].sub.3

wherein φ=phenyl. Of course other nitro/acids can be used.

Hubs can also be formed by reacting the corresponding halo-hub (such atribromobenzene) with aminophenol to form triamine compounds representedby the formula: ##STR23## which can then be reacted with an acidanhydride to form a polycarboxylic acid of the formula: ##STR24##wherein φ=phenyl; the hub being characterized by an intermediate etherand imide linkage connecting aromatic groups. Thio-analogs are alsocontemplated, in accordance with U.S. Pat. No. 3,933,862.

Phenoxyphenyl sulfone arms radiating from a hub with either an amine orcarboxylic acid are also precursors for making multidimensionalheterocycle oligomers of the present invention.

The best results are likely to occur when the hub is cyuranic acid, andwhen a four-functional compound and end-cap monomer are reacted with thehub to form a short armed oligomer having three or six crosslinkingsites. These compounds are the simplest multidimensional oligomers andare relatively inexpensive to synthesize.

Blends of the multidimensional oligomers, comparable to the blends oflinear oligomers, can also be prepared, as will be understood.

The oligomers can be synthesized in a homogenous reaction scheme whereinall the reactants are mixed at one time, or in a stepwise reactionscheme wherein the radiating chains are affixed to the hub and theproduct of the first reaction is subsequently reacted with the end capgroups. Of course, the hub may be reacted with end-capped arms thatinclude one reactive, terminal functionality for linking the arm to thehub. Homogeneous reaction is preferred, resulting undoubtedly in amixture of oligomers because of the complexity of the reactions. Theproducts of the processes (even without distillation or isolation ofindividual species) are preferred oligomer mixtures which can be usedwithout further separation to form the desired advanced composites.

If the linear or multidimensional oligomers include Schiff base or otherconductive linkages, the composites may be conductive or semiconductivewhen suitably doped. The dopants are preferably selected from compoundscommonly used to dope other polymers, namely, (1) dispersions of alkalimetals (for high activity) or (2) strong chemical oxidizers,particularly alkali perchlorates (for lower activity). Arsenic compoundsand elemental halogens, while active dopants, are too dangerous forgeneral usage, and are not recommended.

The dopants apparently react with the oligomers or polymers to formcharge transfer complexes. N-type semiconductors result from doping withalkali metal dispersions. P-type semiconductors result from doping withelemental iodine or perchlorates. Dopant should be added to the oligomeror blend prior to forming the prepreg.

While research into conductive or semiconductive polymers has beenactive, the resulting compounds (mainly polyacetylenes, polyphenylenes,and polyvinylacetylenes) are unsatisfactory for aerospace applicationsbecause the polymers are:

(a) unstable in air;

(b) unstable at high temperatures;

(c) brittle after doping;

(d) toxic because of the dopants; or

(e) intractable.

These problems are overcome or significantly reduced with the conductiveoligomers of the present invention.

While conventional theory holds that semiconductive polymers should have(1) low ionization potentials, (2) long conjugation lengths, and (3)planar backbones, there is an inherent trade-off between conductivityand toughness or processibility, if these constraints are followed. Toovercome the processing and toughness shortcomings common with Schiffbase, oxazole, imidazole, or thiazole polymers, the oligomers of thepresent invention generally include "sulfone" linkages interspersedalong the backbone providing a mechanical swivel for the rigid,conductive segments of the arms.

Because the heterocycle (oxazole, thiazole, or imidazole) linkages arethemselves within the family of conductive or semiconductive linkages,it may be unnecessary to include Schiff base linkages to achieveconductive or semiconductive properties upon doping. That is, conductiveor semiconductive properties might be achieved simply be doping theoxazole, thiazole, or imidazole oligomers.

Linear or multidimensional oligomers can be synthesized from a mixtureof four or more reactants so that extended chains may be formed. Addingcomponents to the reaction mixture, however, adds to the complexity ofthe reaction and of its control. Undesirable competitive reactions mayresult or complex mixtures of macromolecules having widely differentproperties may be formed, because the chain extenders and chainterminators are mixed, and compete with one another.

All reactions should be conducted under an inert atmosphere and atelevated temperatures, if the reaction rate needs to be increased. Thereaction mixture should be well stirred throughout the synthesis.Chilling the reaction mixture can slow the reaction rate and can assistin controlling the oligomeric product.

While para isomerization is shown for all of the reactants, otherisomers are possible. Furthermore, the aryl groups can havesubstituents, if desired, such as halogen, lower alkyl up to about 4carbon atoms, lower alkoxy up to about 4 carbon atoms, or aryl.Substituents may create steric hindrance problems in synthesizing theoligomers or in crosslinking the oligomers into the final composites.

The following examples are presented to illustrate various features ofthe invention.

EXAMPLE I Synthesis of bis(3-methylphenoxyphenyl) sulfone ##STR25##

A one liter bottle fitted with a stirrer, thermometer, Barrett trap,condenser, and nitrogen inlet tube was charged with 88.3 grams (0.82moles) of m-cresol, 286.6 grams of dimethyl sulfoxide (DMSO), 134.8grams of toluene, and 65.3 grams of a 50% NaOH solution. The mixture washeated to 127° C. and the water was removed. The mixture was then heatedto 165° C. to remove the toluene, and was cooled to 110° C. beforeadding 111.7 grams (0.39 moles) of dichlorodiphenylsulfone. The mixturewas heated for 4 hours at 141° C., before the mixture was poured into 3liters of water to crystallize an intermediate. The water was decanted,and 1 liter of 2-propanol was added. This mixture was heated until themajority of the product dissolved. The product was recrystallized,recovered by filtration, washed with 3 liters of water followed by 500ml of 2-propanol, and dried. 167.4 grams of abis(2-methyl-phenoxyphenyl) sulfone product resulted. The melting pointranged from 83°-85° C.

EXAMPLE II Synthesis of bis(3-carboxyphenoxyphenyl) sulfone ##STR26##

A reaction flask fitted with a stirrer, condenser, thermometer, and N₂purge was charged with 100 grams of the product of Example I, 775 gramsof pyridine, and 155 grams of water. The mixture was refluxed andoxidized with 49 grams of KMnO₄, filtered to recover the intermediate towhich 775 grams of 1.8N NaOH solution was added. The mixture wasrefluxed, oxidized, and filtered again. The oxidation steps wererepeated 5 times. The resulting final product had a melting pointranging from about 213.5° to 219° C.

EXAMPLE III Synthesis of the acid chloride of the product obtained inExample II ##STR27##

Twenty grams of the product of Example II was mixed with 61.2 grams ofSO₂ Cl in a reaction flask, fitted with a stirrer, condenser,thermometer, and dry N₂ purge. The mixture was refluxed for 2 hours andthe SO₂ Cl was distilled off. Two hundred ml of benzene was added andthe mixture was refluxed, cooled, and filtered to recover the rawproduct which was recrystallized to a powder. The powder was mixed with200 ml of benzene, refluxed, and cooled to form a precipitate that had amelting range of about 115° to 118° C.

EXAMPLE IV Synthesis of nadic dicapped polybenzoxazole. Formula weightapproximately 4,000 ##STR28##

5.62 grams (10.66 mmoles) of the acid chloride terminated sulfone ofExample III was combined with 2.47 grams (5.3 mmoles) of nadic dicappedacid chloride in CH₂ Cl₂. The mixture of acid chlorides was added usingan addition funnel to a stirred slurry of 2.88 grams (13.3 mmoles) of3,3'-dihydroxybenzidine in dimethylacidamide (DMAC). The mixture wasstirred for 3 hours at room temperature, and then sat under N₂ for 48hours in the atmosphere. The product was recrystallized from CH₂ Cl₂using petroleum ether, was washed with petroleum ether, and washed againwith methanol. The product yield was 82%, and the product had a meltingrange of 220°-245° C.

EXAMPLE V Synthesis of nadic dicapped polybenzoxazole. Formula weightapproximately 2,410 ##STR29##

12.53 grams (23.76 m-moles) of the acid chloride terminated sulfone inExample III was combined with 7.71 grams (35.65 m-moles) of3,3'-dihydroxybenzidine in pyridine. The mixture was stirred undernitrogen in an ice water bath. 11.00 grams (23.76 m-moles) of nadicdicapped acid chloride in CH₂ Cl₂ was added over a 30-minute period, andthe mixture was stirred 3 hours at room temperature.

The product was precipitated in a blender with water, recovered withfiltration, washed with water, and then washed again with methanol. Theproduct was dried under nitrogen, and had a melting point of about 245°C.

EXAMPLE VI General method of preparation of composites for oligomer ofExample IV

The resulting oligomers were impregnated on epoxy-sized T300/graphitefabric style (Union Carbide 35 million modulus fiber 24×24 weave). Thesolution obtained in each Example was coated onto the dry graphitefabric so that 38% by weight of the resulting prepreg would be theoligomer. The prepreg allowed to dry under ambient conditions to lessthan 1% volatile content. The prepreg was then cut into 6×6-inch piecesand stacked to obtain a consolidated composite of approximately0.062-inch. The stack of prepreg was then vacuum bagged and consolidatedunder a bag pressure of 200 psi for 3 hours at 650° F. and for two hoursthereafter in an autoclave.

EXAMPLE VII

A polybenzoxazole oligomer was made using the procedure described inExamples IV and V. Composite panels were fabricated as described inExample VI, and the mechanical properties of each panel were determined.Table I illustrates the mechanical properties.

                  TABLE I                                                         ______________________________________                                        Summary Mechanical Properties                                                 of Polybenzoxazole Oligomers                                                                      Shear Strength                                            Panel Resin         psi (1) at:                                               Example No.                                                                            FMW      Cure T °F.                                                                       RT     450° F.                                                                      650° F.                       ______________________________________                                        Example IV                                                                             4000     650       3460   --    --                                   Example V                                                                              2500     650       4720   4090  2680                                 ______________________________________                                         (1) Short beam shear test method.                                        

While preferred embodiments have been described, those skilled in theart will recognize alterations, variations, or modifications that mightbe made to the embodiments without departing from the inventive concept.The description and examples, accordingly, are meant to illustrate theinvention. The claims should be interpreted liberally in view of thedescription, and should be limited only as is necessary in view of thepertinent prior art.

We claim:
 1. A heterocycle comprising the condensation product of thereaction of a carboxylic acid chloride of the formula: ##STR30## whereinX₁ =--O-- or --SO₂ --,q=--(CF₃)₂ C--, --SO₂, --S--, or --CO--, andφ=phenyl with a difunctional diamine selected from the group consistingof compounds of the formula: ##STR31## wherein R is an aromatichydrocarbon, and Z is selected from the group consisting of --OH, --SH,or NH₂, each Z being attached to a carbon atom adjacent to a carbonhaving a terminal amine group.
 2. The heterocycle of claim 1 wherein theZ=--OH.
 3. The heterocycle of claim 1 wherein Z=--SH.
 4. The heterocycleof claim 1 wherein Z=--NH₂.
 5. An oligomer comprising the condensationproduct of:(a) 2 moles of a phenylimide end cap including a reactiveamine, phenol or sulfhydryl group; (b) n moles of a four-functionalcompound of the general formula: ##STR32## wherein R is a hydrocarbon, Yis selected from the group consisting of --OH, --NH₂, or --SH, Y is on acarbon adjacent to the carbon having an --NH₂, and n is selected so thatthe oligomer has an average formula weight of between about 500-30,000;and (c) n+1 moles of a dicarboxylic acid halide,
 6. The oligomer ofclaim 5 wherein the end cap is selected from the group consisting of:##STR33## R₁ =lower alkyl, aryl, substituted aryl, lower alkoxy,aryloxy, halogen, or mixtures thereof;G=--SO₂ --, --S--, --O--, or --CH₂--; i=1 or 2; j=0, 1, or 2; Z=--OH, --NH₂, or --SH; T=methallyl orallyl; Me=methyl; and φ=phenyl.
 7. The oligomer of claim 6 whereinz=--OH.
 8. The oligomer of claim 6 wherein the four-functional compoundis selected from the group consisting of:dihydroxybenzidine;dimercaptobenzidine; 2,6-diamino-3,5-dihydroxybenzene;2,6-diamino-3,5-dimercaptobenzene; or diaminobenzidine.
 9. The oligomerof claim 6 wherein the dicarboxylic acid halide is selected from thegroup consisting of: ##STR34## wherein X is an acid halidefunctionality, andq=--(CF₃)₂ C--, --SO₂ --, --S--, or --CO--.
 10. Ablend comprising the oligomer of claim 6 and a comparable,noncrosslinking polymer.
 11. A prepreg comprising the oligomer of claim6 and a reinforcing additive in fiber or particulate form.
 12. Acomposite comprising the cured prepreg of claim 6.